Geoscience Reference
In-Depth Information
It is also believed that the rain forest has potential
material for medicines. Almost a quarter of the drugs
prescribed in the United States are derived from tropical
rain forest plants, and at least 2,000 rain forest plants have
been identified by the US National Cancer Institute as
having anti-cancer properties.
shield from intense rainfall and sunlight. Once it is
removed, run-off is increased and erosion rates rise
dramatically, especially on slopes. Severe erosion can strip
off topsoil down to the impermeable lateritic hardpan,
which makes recolonization difficult. Additionally, the
run-off may become concentrated into gullies, which
greatly increase sediment yield into rivers. Logging activity
often generates erosion along their access tracks. In
Thailand in 1988 forty people were killed in mudslides
which were blamed on illegal logging.
Run-off
. The more rapid run-off associated with
deforestation can give rise to flooding downstream of the
affected area. In Venezuela mass wasting caused by
unseasonal floods in December 1999 was accentuated
by development and forest clearance on unstable
mountain slopes near Caracas. It was estimated that up
to 30,000 people died and up to 400,000 were made
homeless as poorly constructed shanty dwellings were
swept down the hillsides into the roaring torrents that
occupied the valleys. Some of this sediment accumulates
Environmental factors
As well as their biological role, the rain forests interact
with their environment and affect the soils, hydrology and
climate. Let us have a look at these aspects in turn.
Soils
. Clearance of trees removes the main source of
nutrients to the forest soils and at the same time allows
rainwater to reach the floor unmodified by the canopy. As
it is very difficult to replace nutrients by the addition of
fertilizer the soils will rapidly become poorer through the
leaching effects of heavy rainfall (
Figure 27.10
).
With complete forest cover the soils are protected from
erosion by root mats; dense ground and decomposing
vegetation act as a sponge and the canopy provides a
VENEZUELA
0
500
FRENCH
GUIANA
SURINAM
km
ATLANTIC
OCEAN
GUYANA
Boa Vista
RORAIMA
COLOMBIA
AMAPA
Macapa
Belém
São Luis
Manaus
AMAZONAS
PARA
MARANHAO
PIAUÍ
ACRE
Porto Velho
TOCANTINS
Rio Branco
RONDONIA
PERU
BAHIA
Production quantity (t)
MATO GROSSO
1 800 000
6 000
Cuiabá
Rice
Soya
Maize
Cattle raising
State capital
Highway
Indigenous land
Conservation area
GOIAS
BOLIVIA
MATO GROSSO DU SUL
Source:
ftp://geoftp.ibge.gov.br
